A polymeric rubber-like elastomer basically has either a soft polymer structure or a structure comprising a rigid polymer segment coupled with a soft polymer segment, and is used in a wide variety of industrial fields because it has rubber-like elasticity at ordinary temperatures and can be mechanically shaped due to its plasticization at high temperatures, as with thermoplastics. Typical polymeric rubber-like elastomers include those based on styrene, olefin, polyester, polyurethane, polyvinyl chloride, and polyamide. These polymeric rubber-like elastomers are usually provided as processed products produced by a mechanical operation such as extrusion molding, and when used in coating materials for various materials; adhesives or tackifiers; binders; modifiers for emulsions or the like; binders for fibers, or the like, the polymeric rubber-like elastomers are used desirably in the form of an aqueous dispersion.
Aqueous dispersions of polymeric rubber-like elastomer have been extensively studied, and an aqueous dispersion of styrene-based rubber-like elastomer has been provided as a practical product. The aqueous dispersion of styrene-based rubber-like elastomer is typically produced by mixing an organic phase solution containing a styrene-based rubber-like elastomer dissolved in an organic solvent, with an aqueous phase solution containing an emulsifier (surfactant) dissolved in an aqueous medium, and emulsifying the mixture with a homomixer or the like, followed by removing the organic solvent (see Patent Documents 1 and 2 below).
As stated above, aqueous dispersions of styrene-based rubber-like elastomer are in practical use. However, processed products obtained from the aqueous dispersions of styrene-based rubber-like elastomer are generally inferior in abrasion resistance, flex resistance, oil resistance, and weather resistance.
A polyamide-based rubber-like elastomer, on the other hand, can provide processed products that are not only excellent in these properties, i.e., abrasion resistance, flex resistance, oil resistance, and weather resistance, but also excellent in transparency, flexibility, impact strength, tensile strength, chemical resistance, and heat resistance, and further has an advantage in that its processed products can be made thin-walled because it exhibits higher stress upon deformation than other polymeric rubber-like elastomers of the same rigidity, and is thus useful, for example, as a material for producing packaging films, automobile parts, sporting goods, and medical devices. Accordingly, there have been demands for aqueous dispersions of polyamide-based rubber-like elastomer; however, such aqueous dispersions of polyamide-based rubber-like elastomer have not been put in practical use so far, and remain to be realized.
Examples of aqueous dispersions of polyamide-based rubber-like elastomer and their production processes considered so far include a process in which a polyamide-based rubber-like elastomer is emulsified and dispersed in the presence of an aqueous medium and a surfactant (see Patent Documents 3 and 4 below).
However, aqueous dispersions of polyamide-based rubber-like elastomer obtained by such a process are not satisfactory in terms of storage stability, particularly when stored at a low temperature. Moreover, for use as a coating material, further improvement is desired in their heat resistance and mechanical properties.